The invention pertains to a process for practicing high performance, cation-exchange chromatography. Materials for performing high speed, liquid chromatagraphic analyses are known where only the thin, outer surface of the chromatographic support material actively exchanges ions with a liquid medium.
Parrish, in Nature 207:402 (1965), describes "superficial ion-exchange chromatography" using beads of cross-linked polystyrene which bear a "shallow surface layer" of ion-exchange groups. The beads Parrish describes there are about 240 microns mean diameter and are surface-sulfonated, according to our calculations, to a depth of approximately 210 Angstroms. Small in U.S. Pat. No. 3,102,782 discloses surface-sulfonated styrene-divinyl benzene copolymer beads ranging from 0.5-8% cross-linking with capacities from which we calculate the depth of surface-sulfonation to be within the range contemplated for utilization in the present chromatographic process. The materials of Small were utilized for solvent extraction of heavy metal salts into a complexing organic solvent phase imbibed by the beads. There is no suggestion that compositions within the range of cross-linking and calculated depth of surface-sulfonation Small teaches would prove extremely useful in the practice of cation-exchange chromatography.
Others have described ion-exchange chromatography with materials having an impervious core, such as a glass bead or diatomaceous earth particle, coated with a skin-like layer of styrene-divinylbenzene resin which is appropriately sulfonated or aminated to produce, respectively, cation or anion-exchange materials, Analytical Chemistry 39:1422; British Pat. Nos. 871,541 and 1,344,706.
To our knowledge however, no previous artisan has noted the critical relation between the degree of cross-linking and the calculated depth of surface-sulfonation for such ion-exchange chromatographic support materials. Indeed, two articles by Hansen et al., J. Chrom. Sci. 12:458-463 and 464-472 (1974) speculate that in 8% cross-linked materials, the depth of the ion-exchanging surface layer should be about 2000 to about 7000 Angstroms to optimize performance in ion-exchange chromatography in resin particles of about 10 to 60 microns particle size. Horvath in J. Chrom. Sci. 7:109-116 (1969) concludes that the smaller the shell thickness in a "pellicular" packing for high pressure liquid chromatography, the smaller the contribution of the packing to reduced plate height of his chromatographic columns.